1. Field of the Invention
The invention relates to a lance for removing deposits adhering to the tube sheet of a steam generator.
2. Background and Relevant Art
Deposits which become hard over time during operation and consist of corrosion products are formed on the tube sheet plate of steam generators in nuclear or conventional power stations. After prolonged operation or if cleaning has not been carried out for a prolonged period, said deposits may become very hard. Very hard deposits may also arise if, in addition to the corrosion products, silicates or alumino-silicates from the secondary circuit are involved in the formation thereof.
Deposits on the tube sheet may cause long term damage to the steam generator by, for example, constricting the steam generator tubes or heating tubes extending away from the tube sheet. The complete removal of hard deposits from the surface of the tube sheet is therefore necessary in order to prevent damage to the steam generator in the long term. This is customarily carried out by high-pressure spraying with water by means of lances which are introduced into the bank of tubes of the steam generator. However, with this known method which is known in the art as inner bundle lancing (IBL) and is known, for example, from DE 100 06 0667 A1, DE 40 39 376 A1 and U.S. Pat. No. 4,424,769 and which operates with water pressures of up to approximately 220 bar, older deposits, deposit bridges between the heating tubes, and silicate-containing deposits of highly compact structure are hardly removed. In these cases, the water jet rebounds or is simply merely deflected, and therefore at most a merely minimal cleaning effect can be obtained. There is currently also no chemical method with which the abovementioned silicate-deposits can be dissolved.